A subject - verb - subject predicative sentence consists of a subject, a copular verb, and an obligatory subject predicative. The subject is always filled by a noun phrase or a nominal clause. The subject predicative (or subject complement) is an element which describes, identifies or defines the subject. The subject predicative is always an adjective phrase or a noun phrase (including nominals).
The subject in the sentence above is a two-word noun phrase (adjective + noun). The verb is the commonest copular verb (be) and the subject predicative is an adjective. This is a description.
The subject in the sentence above is a one-word noun phrase. The verb is the commonest copular verb (be) and the subject predicative is a complex noun phrase (the head is 'plastic', premodified with an adjective 'first' and postmodified with a prepositional phrase 'from synthetic components'. This is identification. These identification/definition constructions require the the verb be and a definite noun phrase. The construction can usually be reversed, unlike a description.
Here are a few more examples (click and drag to stop, start and find the sentence you wish to examine):
You can view an analysis of some of the noun phrases in the above examples in the animations below (click on Subject to view each noun phrase).
Indicates an obligatory element. Indicates an optional element.In the above example the subject phrase is "Many mushrooms". "Many" is a quantifier (one type of determiner) and the head is "mushrooms". The subject predicative is "healthy and delicious" - two adjectives describing "many mushrooms" (not all mushrooms!). This subject predicative falls into the description category.
In the above example the head of the subject phrase is "Tai Chi", a compound noun and there are no modifiers. The subject predicative is a noun phrase and the headword is "exercise". It is premodified by the compound noun "mind-body". This subject predicative falls into the identifier/definition category.
In the above example subject phrase is "Our cities". "Our" is a possessive determiner. The copular verb is "remain". The head of the subject predicative is the adjective "unprepared" which is premodified by the adverb "woefully" and postmodified by the prepositional phrase "for extreme weather". This subject predicative falls into the description category.
In the above example the head of the subject phrase is "engine". The subject predicative is a nominal in the form of a wh-clause. This subject predicative falls into the identifier/definition category. In fact you can turn the sentence around: "What helps the aircraft to fly is the engine."
You can see further examples of this structure (including optional elements) on the SVPs page and test yourself on the SVPs exercise page.